1. Field of the Invention
The present invention relates to a switching element drive circuit and in particular relates to a switching element drive circuit that drives a power switching element of a power converter.
2. Description of the Related Art
The field of application of power converters employing power switching elements has greatly expanded with the use of switching elements of increased capacity and increased speed. Recently, such power switching elements, in particular IGBTs (insulated gate bipolar transistors) and MOSFETs (metal-oxide-semiconductor field effect transistors), which are MOS gate type switching elements, become applicable on a wide scale.
IGBTs or MOSFETs are switching elements of the non-latching type in which the on/off condition does not continue of itself, and have the considerable advantage that higher controllability by the gate drive can be achieved compared with latching type switching elements such as thyristors. Such non-latching switching elements can suppress surge voltage or surge current by gate control, and control the current or voltage gradient in the transitional period of switching, even in the transitional period of turn-on/turn-off switching.
An example of application utilizing the distinctive features of such non-latching switching elements is a multiple series high voltage converter using the active gate drive technique. A multiple series high voltage converter realizes a high-voltage converter that can be employed in high-voltage applications such as power systems, by connecting a large number of elements of limited withstand-voltage in series. A multiple series voltage converter is, however, subject to the problem that large variations in the apportioned voltages are produced by even slight mismatching of the switching timings between the large number of series-connected elements. A way of dealing with this problem is using the active gate drive technique.
In the active gate drive technique, the voltage that is applied between the main electrodes of a power switching element is divided by resistances, so that these divided voltages are used as power sources and current is injected into the gate electrode in accordance with the voltages applied between the main electrodes of the power switching element: an example is disclosed in Japanese Patent Disclosure (kokai) No. 2005-86940.
In the example of this Japanese Patent Disclosure, the gate electrodes, which are the control input terminals of the switching elements, are connected with voltage amplifiers through gate resistances, and are connected also with the output of a control current source. The input of the control current source is connected with the output of the voltage amplifier and the collector/emitter voltage of the switching element obtained by voltage division by the voltage dividing resistance is applied to the input of the voltage amplifier. Thus, in the normal operating condition, the switching element performs on/off operation in accordance with the gate signal that is applied through the voltage amplifier, but, if surge voltage is generated when the switching element is turned off, the output current from the control current source is increased. The current that flows into the gate terminal of the switching element from the control current source increases the gate voltage of the switching element, thereby increasing the collector current of the switching element: as a result, the collector voltage of the switching element drops. Surge voltage of the switching element is suppressed by this action.
In another form of the active gate drive technique, in a power converter wherein IGBTs are connected in series, breakdown of the IGBTs by overvoltage due to voltage imbalance generated between the IGBTs when overcurrent flows through the series-connected IGBTs constituting the power converter is prevented increases the gate voltage when the collector voltage of an IGBT increases, so that, when the collector voltage becomes high, the gate voltage becomes higher than the gate voltage in the steady on condition of the IGBT. An example is disclosed in Laid-open Japanese Patent Disclosure (kokai) No. 2003-69401.
This Laid-open Japanese Patent Disclosure (kokai) relates to an element protection system when a short-circuiting fault occurs in a multiple series power converter consisting of this active gate drive technique. Usually, an IGBT is turned on or off by applying a pulse generated by a pulse generator of a gate circuit to the gate of the IGBT through a comparator. Thus, when a short-circuit occurs in an arm that is paired with an arm constituted by a plurality of IGBTs, the respective IGBTs are protected from excess applied voltage.
Since the same arm is constituted by a plurality of IGBTs, the same arm current flows in each of the IGBTs, so, depending on the variability of the properties of the IGBTs themselves, larger voltage may be applied to a specified IGBT than to other IGBTs. This occurs in an element whose IGBT saturation current value is smaller than that of other elements. When the applied voltage of this specified IGBT exceeds a prescribed value, the value of the voltage divided by the voltage dividing resistance exceeds the output voltage of the pulse generator. The comparator is arranged to output the higher voltage of its two inputs, so, since then, the gate voltage of the specified IGBT becomes a voltage higher than the ordinary on gate voltage. The saturation current value of an IGBT increases as the gate voltage rises, so the saturation current value of the specified IGBT rises, and, accompanying this, the voltage apportioned to the IGBT drops. In this way, the possibility of the voltage applied to the switching element exceeding the fixed value determined by the circuit in the event of a short-circuit is eliminated.
Thus, with the two Laid-open Japanese Patent Disclosure (kokai) s described above, feedback control in the gate drive circuit of the main voltage of the switching element prevent the switching element from being broken. In these systems, the set value of the voltage at which operation of the control circuit commences with respect to the voltage of the externally connected voltage division circuit is determined and the technique is adopted of performing control when the divided voltage is higher than this set value.
However, in the case of a power converter using switching elements, if the applied voltage is a DC voltage, the applied DC voltage may sometimes have a somewhat higher value than the rated voltage. Since, when such DC voltage is applied, protective action must not be performed unless required, the set value for protective action is restricted by the applied DC voltage. In other words, when the active gate technique is employed, the set value is set such that the control action is only performed when the voltage becomes even higher than this high voltage. The set value is therefore a higher value than the rated voltage and the time after surge voltage generated until commencement of control is therefore long. The loss generated in the switching element is thus increased corresponding to the prolongation of the time until commencement of control and it must be controlled in an overvoltage condition.
Next, the problem that arises in the event of short-circuiting will be described.
Usually, when a switching element is turned off, the voltage between the main electrodes of the switching element becomes an extremely low voltage determined by the switching element and the divided voltage also becomes close to 0 V. In contrast, in the event of a short-circuit of the switching element, voltage determined by the switching element and the short-circuit current are generated. Although the short-circuit current is determined by the characteristics of the switching element, this current is an extremely large current greater than the rated current by a factor of up to several times, to several tens of times. if such a large current continues to flow, the switching element is broken in a short time. Cases where the voltage of the voltage division resistance is high even after turn-on must therefore be regarded as cases of short-circuiting and protective action taken.
The voltage that is generated between the main electrodes of the switching element in the event of short-circuiting is low, as compared with the overvoltage level such as would break the switching element.
Consequently, even if voltage of the switching element in the event of short-circuiting is generated, this constitutes only a small change when observed by the voltage division circuit. Therefore there is a risk that the switching element may be broken due to be delayed to detect short-circuit.